This invention relates to silicon and silicon wafers used in the manufacture of semiconductor devices and to a process for improving the minority carrier recombination lifetime in silicon by reducing iron contamination.
The minority carrier recombination lifetime and minority carrier diffusion length of a silicon body are related and are, therefore, sometimes referred to interchangeably. In p-type silicon, minority carrier diffusion length and minority carrier recombination lifetime are related according to the following expression: L.sub.d =((D.sub.n)(t.sub.n)).sup.0.5 wherein L.sub.d is the minority carrier diffusion length in centimeters, D.sub.n is the electron diffusion coefficient, taken to be 35 cm.sup.2 /sec, and t.sub.n is minority carrier recombination lifetime in seconds. In n-type silicon, minority carrier diffusion length and minority carrier recombination lifetime are related according to the following expression: L.sub.d =((D.sub.p)(t.sub.p)).sup.0.5 wherein L.sub.d is the minority carrier diffusion length in centimeters, D.sub.p is the hole diffusion coefficient, taken to be 12.5 cm.sup.2 /sec, and t.sub.p is minority carrier recombination lifetime in seconds.
Various techniques may be used to measure the minority carrier recombination lifetime (or minority carrier diffusion length) of a silicon wafer and typically involve injecting carriers into a wafer sample by means of a flash of light or voltage pulses and observing their decay. One process for measuring minority carrier recombination lifetime is the surface photovoltage (SPV) technique described in Zoth and Bergholz, J. Appl. Phys., 67, 6764 (1990). Alternatively, diffusion length may be measured using an ELYMAT instrument manufactured by GeMeTec (Munich, Germany) which measures to a resolution of about 1 mm the photocurrents generated by a scanning laser beam and collected by a dilute HF electrolytic function. Minority carrier diffusion lengths are calculated from these data and diffusion length images can be generated. See, e.g., H. Foell et al., Proc. ESSDERC Conference, Berlin 1989, p. 44. The calculated diffusion length values are readily converted to minority carrier recombination lifetime values using the formulas above.
As integrated circuit technology advances and the size of electrical devices continues to decrease, the need to impose tighter limits on metallic contamination continues to increase. Metallic contamination in silicon such as a silicon wafer causes a reduction of the minority carrier recombination lifetime and results in spurious electrical current leakage in electrical devices incorporating the silicon. Spurious electrical current leakage paths occur at metallically contaminated point defect sites in the vicinity of electrical junctions within the device or sub-device cell structure, or through leakage paths in the oxides used as field effect gates of isolation structures. Both paths are deleteriously increased in size when the silicon is contaminated with metals such as iron, copper, nickel, chromium, and titanium.
Gettering techniques have been used in an effort to reduce or eliminate metallic contamination in silicon wafers. Such techniques involve the introduction of defect sites into silicon wafers at locations which do not disturb the functioning of electronic devices fabricated in the wafer. For example, defects have been introduced to the backsurface of the wafer (external gettering) and by allowing oxygen to precipitate in the bulk of the wafer but not near the surface (internal gettering).
Although gettering techniques have been useful in preventing fast diffusing metals such as copper and nickel from actively participating in recombination processes in those areas containing electronic devices, such techniques have not been particularly effective for metals such as iron, chromium and titanium, which are relatively slow diffusing as compared to copper and nickel. Additionally, the defects introduced into silicon as gettering sinks can themselves reduce the quality of silicon, for example, by reducing minority carrier recombination lifetime. As a practical matter, therefore, industry has to date had little choice but to identify and eliminate to the extent possible sources of iron, chromium and titanium, particularly iron, contamination. Such an approach, however, is becoming increasingly costly as it becomes necessary to achieve ever decreasing levels of iron contamination; the manufacture of silicon wafers and devices having silicon components includes many steps and potential sources of contamination which must each be carefully examined and controlled.